influence of micro gaps on the magnetic characteristics of fi-relay · 2018-12-05 · coating...

• AC/DC module with Magnetic Fields Stationary Study was applied for calculations

• Thin Low Permeability Gap feature was used to model coating layer on anchor‘s bottom surface and to model functional gap between S-yoke and L-yoke

• Given BH-curve for softmagnetic parts was used • Relay impedance for demagnetized relay was

calculated at referenced input coil current Icoil=4.24mA

• Magnetic flux in magnetized relay was calculated

RESULTS:

Magnetic flux over anchor drops significantly, if anchoris coated with 1µm of non-ferromagnetic layer. We canuse higher magnetization of magnet to compensatethat drop, but FI-relay‘s impedance is lower thanrequired.The solution is to change the coil and increase numberof its turns. COMSOL® results were found useful andpositively validated on prototypes.

Figure 3. Magnetic flux density at Icoil=4.24mA and N=423 turns

Influence of Micro Gaps on the Magnetic Characteristics of FI-RelayL. Strehar, D. Janc, A. Smrkolj, N. Štrovs, A. Pušnik

R&D ETI d.o.o., Izlake, Slovenija

FI-relays are built in protective residual current devices(RCD). They tripp at appearance of residual currents(i.e. earthleakage current) and consequently breakmain contacts of RCD. FI-relays must act reliably at verylow tripping electrical power (μVA). They are made ofsoftmagnetic parts called anchor and yokes, permanentmagnet, coil, spring and some plastic parts.

This research is about partial reconstruction of ETI'sexisting FI-relay. We would like to achieve higher outputmechanical forces (work) on FI-relay's striking pin, butits electrical tripping power P[VA] and impedance Z[]must remain the same. Therefor we redesigned anchorpositioning and replaced existing spring with torsionspring. To decrease mechanical deformation on anchorcontact surface that limits required lifecycle test, weused additional thin non-ferromagnetic coating withhigh hardness properties on bottom anchor side.Coating presents new added micro air gaps betweenanchor and yokes and influences changes of relay'smagnetic characteristics. We used AC/DC module toexamine magnetic effects of coating with differentthickness.

Figure 1. Existing and a prototype of new ETI‘s FI relay with coated anchor

Figure 2. Presentation of FI-relay model for AC/DC Analysis

Figure 4. Calculated impedanceas a function of gap thicknessat Icoil=4.24mA and N=423 turns

Existing FI-relay has an effective micro gaps about0.6um between each polished magnetic pole. Initialimpedance is 64.3. When it‘s added non-ferromagnetic coating with required thicknes 1µm,impedance drops to 44.2 (Figure 3.).

INTRODUCTION:

COMPUTATIONAL METHODS:

Figure 5. Magnetic flux density in relay generated by permanent magnent for Br=0.15T

Magnetic flux generated by permanent magnet isdivided in magnetic flux 1, which holds anchor inposition over poles, and magnetic flux 2. Their valuesdepend on how high is magnet‘s remanent flux densityand also on level of saturation in magnetic fluxconducting softmagnetic parts.

Figure 6. Existing relay - calculated values of magnetic flux 1 and 2 [µWb]as a function of magnet‘s remanent magnetic flux density Br[T]

CONCLUSIONS:

23.5mm

Excerpt from the Proceedings of the 2018 COMSOL Conference in Lausanne